In the solvent extraction method used in the hydrometallurgical recovery of metals, two mutually insoluble or poorly soluble solutions are first mixed together in a mixing stage to form a dispersion. After this the dispersion is routed to a settling section, in which the dispersion generally flows horizontally and the purpose is to separate the dispersion again into two pure layers one on top of the other. In the actual extraction stage the aqueous solution contains one or several valuable metals and impurities, and the purpose is to achieve mass transfer during the extraction stage, in other words to transfer one or more of the desired valuable metals into the organic extraction solution, leaving the impurities in the aqueous solution. In the stripping stage the organic extraction solution is contacted with another aqueous solution and the purpose is to obtain the reverse mass transfer, i.e. to transfer one or more valuable metals from the organic extraction solution into the aqueous solution, which is subsequently usually routed to metal recovery by electrowinning.
In order to mix an aqueous solution and an organic extraction solution into each other in the mixing section of an extraction cell, a radial turbine is often used. Another mixer type for the extraction cell mixing space known in the prior art is a mixer consisting of tubular coils and their support arms, which is described for example in U.S. Pat. No. 5,185,081. This mixer is made up of two tubular coils rising upward and the support arms attaching the coils to the mixer shaft. The tubular coils make about two rotations around the shaft. On the basis of the drawings in the publication, it can be deduced that the pitch angle of the coils is in the region of 10 degrees. Straight support rods are an essential part of the mixer. The lower support rods are horizontal, the central rods are inclined upwards towards the shaft at an angle of about 30° and the topmost rods are inclined downwards towards the shaft at an angle of 30°. The mixer may also consist of two pairs of spiral tubes side by side.
A vertical rotation is obtained in the solution by means of the mixer in question and the support rods, particularly the topmost support rods, bring about radial mixing in the solution. The mixer also includes in its upper section a flow reverser placed around the shaft, and this enables the direction of the solution to be diverted downwards in the central part of the mixing space. It is also typical of the mixer that its diameter is 0.5-0.75 times the diameter of the mixing space and the height 0.6-0.9 times the effective height of the mixing space. The mixer is of the up-pumping type. In this way an ascending flow is formed in the mixing space on the periphery, which is turned downwards in the upper part of the space by means of the mixer towards the bottom part of the mixing space. Using and sustaining the vertical circulation evens out the mixing intensity of the mixing space. The mixer support rods bring about a droplet size distribution, especially in the upper section of the mixing space, that is not the most advantageous for forming a dispersion in solution extraction, even though it is considerably better than that obtained with a turbine or blade mixer.